1) Field of the Invention
This invention relates generally to fabrication of semiconductor devices and more particularly to the fabrication of a shallow trench isolation (STI) structure and most particularly to a shallow trench isolation (STI) structure with an air gap.
2) Description of the Prior Art
Shallow trench isolation (STI) are commonly used in the fabrication of semiconductor devices to isolate active regions of the semiconductor substrate. Usually, the isolating trench is filled with a dielectric material. Polysilicon, chemically vapor deposited (CVD) silicon dioxide, BSG glass, and polyimides are examples of prior art trench-fill materials. Each of these materials have a dielectric constant which is considerably larger than that of air.
Present conventional shallow trench isolation (STI) structure use high density plasma chemical vapor deposition (HDPCVD) oxides as the filler material. As the K value of the silicon oxide is about 4.0, channel stop implants are usually required to raise the field inversion voltage. As will be appreciated by those skilled in the art, an air-filled trench would be advantageous in that it would minimize the parasitic coupling between devices isolated by the trench and reduce electrical leakage and mechanical stress as compared with trench-fill materials of the type described above. Proposals have been made in the prior art for an air-filled isolation trench.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering U.S. Pat. No. 6,071,805 (Liu) shows an air gap between lines. The air gap formed by decomposing an organic material filler. U.S. Pat. No. 5,387,538 (Moslehi) shows an air gap lateral to a STI. U.S. Pat. No. 5,953,626 (Hause et al.) shows an air gap for conductive lines. U.S. Pat. No. 6,001,705 (Zombrano) and U.S. Pat. No. 5,508,234 (Dusablon, Sr. et al.) shows STI processes with air gaps. U.S. Pat. No. 5,098,856 (Beyer) shows a process to form air gaps in STI by removing a sacrificial layer.
However, further improvement in air gap STI structures are needed.
It is an object of the present invention to provide a method for fabricating a shallow trench isolation (STI) structure.
It is an object of the present invention to provide a method for fabricating a shallow trench isolation (STI) structure having an air gap.
It is an object of the present invention to provide a method for fabricating a shallow trench isolation (STI) structure having an air gap formed by vaporizing a filler material through a permeable cap layer.
It is an object of the present invention to provide a method for fabricating a shallow trench isolation (STI) structure having an air gap formed by vaporizing a filler material through a permeable cap layer using an insitu process.
To accomplish the above objectives, the present invention provides a method of manufacturing a shallow trench isolation (STI) with an air gap which is characterized as follows. A pad layer and a barrier layer are formed over the substrate. The pad layer and the barrier layer are patterned to form a trench opening. We form a trench in substrate by etching through the trench opening. A first liner layer is formed on the sidewalls of the trench. A second liner layer over the barrier layer and the first liner layer. A filler material is formed on the second liner layer to fill the trench. In an important step, a cap layer is deposited over the filler material and the second liner layer. The filler material is heated to vaporize the filler material so that the filler material diffuses through the cap layer to form a gap. An insulating layer is deposited over the cap layer. The insulating layer is planarized. The barrier layer is removed.
Key elements of the present invention are the cap layer that the filler layer diffuses through to create the air gap. The filler material is covered by the cap layer and is not exposed to the atmosphere. This is a critical difference compared to the prior arts. Also, the filler material is vaporized/decomposed by a HDPCVD plasma process at an elevated temperature. Preferably the filler material is polybutadine (PB).
Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of instrumentalities and combinations particularly pointed out in the append claims.